1. Field of the Invention
This invention relates to cameras having electrically timed shutters, and more particularly to camera with an electrically operated magnetic device controlling the opening and closing operation of the shutter mechanism whereby when the voltage of the electrical power supply source for the device falls below a prescribed level, the camera is positively returned to the wound up position so that the shutter is prevented from being accidentally left open.
2. Description of the Prior Art
Conventional methods of locking camera shutter blades mechanically after charging the shutter when the camer is wound up, followed by releasing the shutter from its locking connection have been realized in the following three types of electrically operated magnetic control devices for the shutter:
(1) During the period of energization of an electromagnet, the locking holds. Then when the duration of energization is terminated, the shutter is released from the locking connection by the force of a previously power-stored or charged spring;
(2) The electromagnet is combined with a permanent magnet so that when the electromagnet is not energized, the attractive force of the permanent magnet holds the locking connection. Then when the electromagnet is energized, the magnetic flux of the permanent magnet is cancelled out, permitting the force of the previously power-stored spring to release the shutter; and
(3) Energization of the electromagnet causes attraction of an armature which in turn causes releasing of the shutter.
Considering particularly the closing operation of the shutter, type (1) has a disadvantage that because of the necessity of maintaining a current supply all the time when the shutter is open, the consumed electrical energy is large particularly in the case of long exposures.
Further, the types (2) and (3) suffer from the possibility of the accidental occurrence that the shutter is left open should the voltage of the battery fall at a time during the exposure, and the magnetic control device is not actuated by the current supply to the electromagnet after the termination of duration of the predetermined shutter time. If the operator is unaware of this accident, then when the film is rewound, all the exposed area of the film will be fogged. Nevertheless, until now, no good idea that can solve these problems has been put into practice.